Realistically the evolution of early firearms took hundreds of years whereas I get the impression that the Civ 5 renaissance musketman belongs in the 1700s then riflemen in the latter 1800s etc...
perhaps there should be an arquebus unit between longswordmen and musketman. I would give the unit the same strength as a longswordman but a much reduced production cost - firearms allowed armies to be raised much more quickly and cheaply rather than the long training times it would take to master swords and longbows etc.
This way an arquebus doesn't immediately obsolete either a longswordsman or a pikeman but the cheaper cost should encourage a gradual transition from melee combat to gunpowder. Then musketmen follow on as the next stronger unit at a later tech.
Historically, the first effective gunpowder weapon was there giant 'Bombard' which immediately made all city/castle walls obsolete. Far more important than the Trebuchet in the development of fortifications and armies.
The Civ 'Musketman' seems to (from the graphics and its placement in the Tech Tree) represent the matchlock musket or Arquebus of the 1500 - 1680 period. The factors match, also - it required pikemen to defend it from cavalry, since the clumsy weapons wouldn't take a bayonet.
What's been missing from all of the Civ games is the smoothbore flintlock musket with bayonet, which dominated combat from about 1700 to 1840 or so. They had twice the rate of fire, could have much tighter formations, and with the socket bayonet, could defend themselves quite well against cavalry, thus making the pikeman obsolete by the early 1700s. The 'Rifleman' in Civ really represents the breech loading gunpowder rifle of the 1860s like the US Spencer or Springfield and the German Needlegun or French Chassepot.
I've proposed several times that 'Gunpowder' give you the Bombard and the Musket, with the Bombard having 0 effect against units (they took hours to reload) but massive effect against cities, while the Musket has slightly better combat factors than crossbows or longswords, but is much cheaper (it was much simpler, from a metallurgical standpoint, to manufacture a smoothbore musket barrel than to forge a spring steel crossbow or interlocking steel plate armor!).
About 150 - 200 game years later, in the late Renaissance (actually, mid-Enlightenment Era, if we want to add Eras) you get the Fusilier unit, which has much better combat factor than the Musket, has a 25% or more increased effect against cavalry. Promotion-wise, both the Pikeman and the Musketman would promote to Fuslier, and the infantry line then becomes a single line through Rifleman, Magazine Rifle Infantry, Infantry to Mechanized Infantry.
This would also allow Unique Units to claim their proper place: The Carolean was a Fuslier, as was the classic British/English Redcoat.